Drying oven for printed sheets

ABSTRACT

A drying oven for screen-printed sheets has an upright housing divided by a vertical partition into a hot-air chamber and a cold-air chamber. An ascending tray elevator in the hot-air chamber and a descending tray elevator in the cold-air chamber, each comprising two pairs of endless chains, are driven in alternation with a pair of horizontal conveyors at the top and the bottom of the housing whereby trays loaded with sheets to be dried move upwardly in the first chamber, through jets of heated air issuing from an outer wall thereof, and then downwardly in the second chamber through jets of cold air similarly issuing from an outer wall of the latter. The intervening partition serves as a suction duct for aspirating air from both chambers and returning same after heating to the first chamber; the cold air for the second chamber is taken from the surrounding atmosphere.

United States Patent 1191 Colapinto Oct. 14, 1975 [21] Appl. No.: 487,744

Primary Examiner-Carroll B. Dority, Jr. Assistant ExaminerHarold Joyce Attorney, Agent, or Firml(arl F. Ross; Herbert Dubno [57] ABSTRACT A drying oven for screen-printed sheets has an upright housing divided by a vertical partition into a hot-air [52] US. Cl. 34/55; 34/149, 34/190; cha ber and a cold-air chamber, An ascending tray 233 elevator in the hot-air chamber and a descending tray [51] Int. Cl. F26B 11/18 elevator in the cold-air chamber, each comprising two Field of Search 189, 21 pairs of endless chains, are driven in alternation with a 20 pair of horizontal conveyors at the top and the bottom R, 28 of the housing whereby trays loaded with sheets to be dried move upwardly in the first chamber, through jets References Cited of heated air issuing from an outer wall thereof, and UNITED STATES PATENTS then downwardly in the second chamber through jets 1 502 I86 7 1924 Greenwood 34 190 of air Similarly issuing fmm outer wall of the l:675:260 6/l928 Edmunds 34 190 latter The intervening Partition Serves as a Suction duct for aspirating air from both chambers and return- FOREIGN PATENTS l APPLICATIONS ingsame after heating to the first chamber; the cold 496,374 11/1938 United Kingdom 34/189 i f the Second h b i taken f h rounding atmosphere.

10 Claims, 17 Drawing Figures /9 #2 52, 3/ ,/2'/& ,/6

"-r s i'iif /20 US. Patent Oct.14,1975 Sheet10f9 3,911,593

ELECTROMAGNETIC CLUTCH 225 Sheet3 of9 3,911,593

US. Patent 0a. 14, 1975.

U.S. Patent Oct. 14, 1975 Sheet 4 of9 3,911,593

US. Patent 0a. 14, 1975 Sheet 5 of9 3,911,593

Sheet 7 of 9 3,911,593

U.S. Patent Oct. 14, 1975 US. Patent Oct. 14,1975 Sheet80f9 3,911,593

U.S. Patent Oct. 14, 1975 Shegt 9 of 9 3,911,593

DRYING OVEN FOR PRINTED SHEETS FIELD OF THE INVENTION My present invention relates to a drying oven for printed sheets, more particularly for sheets treated by a screen-printing process.

BACKGROUND OF THE INVENTION Screen-printed patterns do not dry as rapidly as those imprinted by a typographic or lithographic process, not only because there is a heavier layer of ink but also because the ink must be of a slow-drying type in order not toclog the interstices of the screen. For the mass production of such prints, therefore, it is important to provide means for expediting their drying so as to avoid the development of bottlenecks in the plant. Conventional drying ovens used for this purpose, working with hot air or with infrared radiation, are relatively cumbersome and cannot keep pace with high-speed screen-printing machines.

OBJECT OF THE INVENTION The object of my present invention, therefore, is to provide an improved drying oven adapted to handle a continuous flow of freshly printed sheets at a high rate.

SUMMARY OF THE INVENTION This object is realized, in accordance with my present invention, by the provision of an upright frame provided with a vertical partition which divides its space into a first and a second chamber, these chambers respectively containing an ascending elevator and a descending elevator each provided with supports for sheet-carrying trays. An upper horizontal transporter at the top of the frame and a lower horizontal transporter at the bottom of the frame serve to convey successive trays from the ascending elevator in the first chamber to the descending elevator in the second chamber and vice versa. The oven housing is provided near the bottom of its first chamber with a loading conveyor for sheets to be dried and near the bottom of its second chamber with an unloading conveyor for the dried sheets. The trays ascending in the first chamber are subjected to jets of hot air whereas the trays descending in the second chamber are subjected to jets of cold air. The hot air is advantageously aspirated from inside the frame, preferably from both chambers thereof, while the cold air is taken from the surrounding atmosphere.

With the movement of the trays on the elevators and on the horizontal transporters properly timed, preferably through the use of common drive means therefor, the trays circulate in an unbroken succession through the housing past the hot and cold jets which advantageously are emitted by perforated panels forming part of a pair of hollow chamber walls. The aspiration of air from within the housing for heating and recirculation to the first chamber may take place by way of the central partition designed as a duct with openings toward -both chambers. In addition, the frame should be provided with exhaust means for vapors released in either chamber.

Each of the tray elevators in the two chambers may comprise two pairs of parallel endless chains confronting one another, the chains of each pair being bridged by horizontal rails serving as tray supports. With the confronting chain runs moving in the same direction,

i.e. up in the first chamber and down in the second chamber, the trays will keep their horizontal position while ascending and descending. As the generally rectangular trays rest with their minor sides on these rails, they can be readily slid off on being engaged from above by depending lugs on a pair of parallel endless chains forming part of the upper transporter and on being engaged from below by upstanding lugs on another pair of parallel endless chains forming part of the lower transporter; it is merely necessary that the separation of the aligned rails on the two elevators be substantially less than the rail length in order that the trays may safely slide from one pair of rails to the other.

For this mode of operation it is advantageous that the horizontal transporters be stationary while the elevators are moving and vice versa. Since the vertical distance between the tray-supporting rails of an elevator will usually be substantially smaller than the path to be traveled by a tray in passing from one elevator to the other, the elevators will generally be driven during a considerably smaller fraction of an operating cycle than the horizontal transporters. In each such operating cycle I prefer to include a pause during which neither the elevators nor the transporters are in motion, this pause allowing for the loading and unloading of the trays. An electromagnetic clutch in the drive mechanism may be operated manually or by the oncoming sheets, e.g. with the aid of sensitive switches, to adapt the operating speed of the drying oven to that of the printing machine by synchronizing the start of a trayadvancing cycle with the completed loading of an empty tray with a freshly printed sheet.

BRIEF DESCRIPTION OF THE DRAWING The above and other features of my invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic side-elevational view of my improved drying oven, illustrating the elevators and the horizontal transporters thereof;

FIG. 2 is a longitudinal sectional view taken on the line II II of FIG. 3;

FIG. 3 is a cross-sectional view from above taken on the line III III of FIG. 2;

FIG. 4 is a similar cross-sectional view taken on the line IV IV of FIG. 2;

FIG. 5 is a sectional side view taken on the line V V of FIG. 2;

FIG. 6 is a top plan view of a tray used in the apparatus of FIGS. 1 5;

FIG. 7 is a front view of the tray shown in FIG. 6;

FIG. 8 is a cross-sectional detail view, drawn to a larger scale, taken on the line VIII VIII of FIG. 5;

FIGS. 9 and 10 are sectional side views respectively taken on the lines IX IX and X X of FIG. 8;

FIGS. 11 14 are diagrams illustrating successive phases in the operation of the system;

FIG. 15 is a schematic front-elevational view showing the air-circulating system of the drying oven;

FIG. 16 is a top view of the assembly of FIG. 15; and

FIG. 17 shows details of a stripper forming part of the unloading mechanism, this being an enlarged representation of the area XVII of FIG. 1.

SPECIFIC DESCRIPTION The drawing shows an oven 1 for the forced drying of a series of sheets, such as those coming from a screen-printing machine, arriving in close succession over an input conveyor 6. The oven frame comprises a base 2 with a superstructure 3 and uprights 4 rising therefrom, terminating at the top in a lintel 5; the entire frame structure supports and is surrounded by a housing wall which has been omitted for the sake of clarity. An entrance slot in this housing wall, just above the level of a support 7 for conveyor 6, enables the introduction of the incoming sheets into a housing chamber 9 in which they are subjected to jets of hot air. Another housing chamber 10 is separated from chamber 9 by a vertical partition 102, 115 (see FIGS. and 16), this partition resting on the superstructure 3 and terminating below the lintel 5. An output conveyor 8 passes through another slot in the housing wall to unload the dried sheets from the apparatus.

Superstructure 3 is upwardly open to give passage to a circulating set of trays 15 forming an ascending stack in chamber 9 and a descending stack in chamber 10. An ancillary conveyor 6' in line with input conveyor 6, operating at a higher speed, serves to' transfer the oncoming sheets onto the trays successively aligned therewith. Conveyors 6 and 6, as best seen in FIG. 4, consists of transversely spaced endless bands which are interleaved with similarly spaced bars 15" forming the sheet-supporting surface of the trays 15', see also FIGS. 6 and 7. The bars 15 rest on two beams 15 which extend parallel to the major edges of the rectangular tray outline and terminate in a pair of angle brackets 15. Conveyor 8, which extends below the level of the lowest tray, is a solid endless web. The motion of the trays has been indicated in FIG. 1 by an upwardly pointing arrow A in chamber 9, a downwardly pointing arrow B in chamber 10, a rightwardly pointing arrow C at the top of the frame 1 and a leftwardly pointing arrow D at the bottom of that frame.

This circulating motion is brought about by an ascending elevator 11 in chamber 9, a descending elevator 11' in chamber 10, an upper horizontal transporter 16 above partition 102, 115 and a lower horizontal transporter 17 beneath that partition. The number of circulating trays advantageously ranges between about 30 and 50.

Ascending elevator 11 comprises a pair of parallel endless chains 211, 212 on one side of chamber 9 and an identical pair, as indicated in FIG. 5, on the opposite side, the two pairs confronting each other with their ascending runs and being mounted on sprocket wheels supported by respective upper and lower shafts 12, 13. In an analogous manner, elevator 11' comprises two pairs of parallel endless chains 213 and 214 confronting each other with their descending runs; these chains are engaged by sprockets on upper and lower shafts 12' and 13. The chains of each pair are bridged by rails which have been designated 14 for the ascending elevator l1 and 14 for the descending conveyor 11. It will be noted from FIGS. 2 and 3 that the gaps separating the rails 14 and 14 of the two conveyors are substantially smaller than the length of each rail.

The upper transporter 16 comprises two endless horizontal chains 18, 18 disposed along the common sidewalls of chambers 9 and 10, these chains being engaged by sprockets on a pair of parallel shafts l9 and 20. A pair of similar chains 18" and 18, forming part of the lower transporter 17, pass around sprockets on shafts 19 and 20' within superstructure 3. Chains l8 and 18 carry lug 16 engageable from above with the trays rising in chamber 9 for shifting them over into chamber l0. Lugs 17' on chains 18" and 18" are engageable with the leading beams 15 of the descending trays in chamber 10 for returning them to chamber 9. A common drive mechanism for all these chains, generally designated 21, is mounted atop the frame 2 5 and has been more fully illustrated in FIGS. 8 10.

Drive mechanism 21 comprises a power shaft 23, driven by an electric motor 223 (FIG. 3), which terminates in a worm 22 meshing with a worm gear 24. The hub 24' of this worm gear is integral with a pair of generally disk-shaped drivingelements 25 and 27 respectively coacting with similar drive elements 26 and 28 coaxially mounted on a shaft 29. Disk '26 is keyed to shaft 29 which, through two sets of bevel gears generally designated 30 in FIGS. 2 and 3, turns the upper shafts l2 and 12' of elevator chains 211 214. Disk 28, which is idle on shaft 29, is positively coupled with a sprocket gear 31 linked via a chain 32 and a sprocket gear 33 with the sprocket shaft 19 of upper transporter 16, the latter being in turn connected via a chain 219 with one of the two sprocket shafts 19' of the lower transporter 17.

FIG. 9 shows details of the transmission 27, 28 serving to drive the horizontal transporters 16 and 17. This transmission is essentially'a Geneva motion, with positive locking of driven element 28 in its rest position which coincides with a rotation of element 27 through an arc of 120. Disk 27, turning counterclockwise (arrow I-I), carries studs 27a adapted to mesh with peripheral recesses 28a of disk 28; the latter, in turn, carries studs 28b adapted to mesh with peripheral recesses 27b of disk 27. The number and distribution of these studs and recesses are so chosen that disk 27, on turning through an arc of 240 corresponding to two thirds of a cycle, rotates the disk 28 clockwise (arrow I-I') through a full turn.

FIG. 10 shows the transmission 25, 26 for the elevators 11 and 11' as a similar Geneva motion, with studs 25a on disk 25 fitting into recesses 26a of disk 26 and with studs 26b on the latter disk fitting into a recess 25b of disk 25. Disk 25 entrains the disk 26 in a clockwise sense (arrow F) through a quarter turn while rotating counterclockwise (arrow F) through an arc of this transmission being thus idle for three quarter of a cycle. The two transmissions of FIGS. 9 and 10 are so interconnected that the operating phase of transmission 27,

28, i.e. the movement of the horizontal transporters during 240 of a 360 cycle, immediately follows the operating phase of transmission 25, 26, i.e. the movement of the elevators during the first 90 of that cycle. This leaves a standstill period of 30, or l/12"' of a turn, in which the trays are stationary and the loading and unloading operations can be performed.

FIG. 3 shows an electromagnetic clutch 225 inserted between drive motor 223 and shaft 23. Clutch 225 may be opened automatically at the end of a cycle (i.e. during the rest period of 30) and may then be reclosed by a nonillustrated sensitive switch tripped by an oncoming sheet on conveyor 6; clutch 225 could also be controlled manually, e.g. for temporarily halting the tray drive during a pause in the printing operation. Particularly in the latter instance the speed of motor 223 may be made adjustable to let the dryer keep pace with the printer.

In FIGS. 11 14 I have illustrated the movement of the sheets through the apparatus in the several phases of a cycle. FIG. 11 shows an oncoming sheet N on conveyor 6 aligned with an empty tray 15 at the lowest level of the ascending stack in chamber 9 whose remaining trays are all loaded. The sheet N on the bottom tray of the descending stack in chamber 10 has come to rest against a stripper 224 whose operation and structure will be more fully described hereinafter with reference to FIG. 17. The bottom section of the ascending elevator in chamber 9 and the top section of the descending elevator 10 carry no trays at this point.

In FIG. 12 the top tray has been shifted from chamber 9 to chamber 10 while the bottom tray has been transferred from the latter chamber to the former, with unloading of its sheet N by the stripper 224 onto the output conveyor 8.

As shown in FIG. 13, conveyor 6 next delivers a fresh sheet N to the previously empty tray at the secondlowest level of chamber 9 even as conveyor 8 carries off the outgoing sheet previously deposited thereon; these operations occur during the standstill phase representing the final 30 of rotation of worm gear 24.

FIG. 14 shows a new sheet arriving over conveyor 6 as the elevators in chambers 9 and 10 raise and lower their respective stacks to restore the initial position of FIG. 11.

FIG. 15 and 16 illustrate the means for circulating hot and cold drying air through the system. The two sections 102, 115 of the central partition are hollow and constitute ducts for the aspiration of air from both chambers, in about equal parts, in response to suction from a pair of blowers 101, 114 connected thereto via a conduit 103, 104 and a conduit 116, respectively. Blower 101 returns the aspirated air from the upper chamber halves, which has passed through an electric heater 105, to a flat duct 106 forming an outer end wall for chamber 9, this duct being bounded by a perforated panel with a multiplicity of apertures from which jets J of hot air issue into chamber 9. Vapors exhausted by blower l 14 from the lower halves of the two chambers are expelled via a discharge pipe 117.

Two other blowers 107, 108 take in fresh air from the atmosphere through a pair of filters 109, 110. Blower 107 works into a flat duct 111, forming an outer end wall of chamber 10, which is similar to duct 106 and is bounded by a perforated panel emitting a multiplicity of cold-air jets J into that chamber. Blower 108 supplies a riser pipe 112 terminating in a horizontal tube 113 which directs jets J" of cold air from above onto the sheet traveling with the uppermost tray from chamber 9 to chamber 10, thereby preventing a detachment of this sheet from its supporting surface.

Stripper 224, more fully illustrated in FIG. 17, comprises a multiplicity of transversely spaced rake teeth 224a fitting between the bars 15" of an oncoming tray 15, moving in the direction of arrow G, while just clearing its supporting beams 15. These teeth are mounted on a horizontal rod 224b whose two halves, held together by nonillustrated bolts, are joumaled on an axle 226 and define a camming groove 224C normally engaged by a spring-loaded ball 227 of a detent 228 supported by a stationary bracket 229. If an extraneous object lands on a tray 15 or comes to lie between the bars 15" thereof, rake teeth 224a are deflected in a clockwise sense so that ball 227 is cammed out of its seat, displacing a switch arm 230 of an alarm device 231 which thereupon halts the operation of the tray drive to prevent possible damage to the mechanism.

I claim:

1. A drying oven for printed sheets, comprising:

a housing including an upright frame provided with a vertical partition dividing the interior thereof into a first and a second chamber;

ascending elevator means in said first chamber and descending elevator means in said second chamber each provided with support for trays having spaced-apart sheet-carrying bars;

upper horizontal transport means at the top of said frame for carrying trays in a direction parallel to said bars from said ascending elevator means to said descending elevator means;

lower horizontal transport means at the bottom of said housing for carrying trays in a direction parallel to said bars from said descending elevator means to said ascending elevator means;

loading means at the bottom of said first chamber for delivering sheets to the trays at the beginning of their ascent;

unloading means at the bottom of said second chamber for removing sheets from the trays at the end of their descent, said unloading means including a stripper member rotatable about an axis transverse to said bars and provided with dependent teeth fitting between said bars;

first jet means in said first chamber trained upon the ascending trays and connected to a source of hot air;

second jet means in said second chamber trained upon the descending trays and connected to a source of cold air;

common drive means for said ascending and descending elevator means and for said upper and lower transport means;

detent means yieldably engaging said stripper member for normally holding said teeth in their dependent position; and

alarm means responsive to a deflection of said teeth from their dependent position by an extraneous object lodged between said bars for deactivating said drive means.

2. A drying oven as defined in claim 1 wherein said drive means comprises a pair of ganged driving elements and a pair of driven elements respectively coupled with said driving elements for intermittent entrainment thereby in different phases of an operating cycle, one of said driven elements being coupled with said elevator means, the other of said elements being coupled with said transport means.

3. A drying oven as defined in claim 2 wherein said driven elements are so coupled with said driving elements as to be simultaneously stationary during a fraction of an operating cycle.

4. A drying oven as defined in claim 1 wherein said source of hot air comprises first blower means for aspirating air from the interior of said housing and driving the aspirated air through a heating zone to said first jet means, said source of cold air comprising second blower means for aspirating air from the surrounding atmosphere and driving same to said second jet means.

5. A drying oven as defined in claim 4 wherein said partition has a hollow upper section and a hollow lower section open toward both chambers, said first blower means communicating with said upper section for aspirating air therefrom, further comprising vapor-exhaust means communicating with said lower section.

6. A drying oven as defined in claim 4, further comprising third blower means for aspirating atmospheric air and discharging same downwardly from the top of said frame for holding a sheet onto a tray conveyed by said upper transport means.

7. A drying oven as defined in claim 1 wherein said trays have sheet-supporting bars spaced apart in a direction said loading means includes an input conveyor with a plurality of spaced-apart endless bands fitting between the bars of said trays.

8. A drying oven as defined in claim 1 wherein said ascending and descending elevator means each comprises two pairs of endless chains confronting each other, the chains of each pair being bridged by traysupporting rails, said upper transport means comprising another pair of endless chains with depending lugs engageable from above with an ascending tray, said lower transport means comprising a further pair of endless chains with upstanding lugs engageable from below with a descending tray, the rails of said ascending and descending elevator means being separated by distances substantially less than the length of said rails.

9. A drying oven for printed sheets, comprising:

a housing including an upright frame provided with a vertical partition dividing the interior thereof into a first and a second chamber, said partition having a hollow upper section and a hollow lower section open toward both chambers;

ascending elevator means in said first chamber and descending elevator means in said second chamber each provided with supports for sheet-carrying trays; upper horizontal transport means at the top of said frame for carrying trays from said ascending elevator means to said descending elevator means;

lower horizontal transport means at the bottom of said housing for carrying trays from said descending elevator means to said ascending elevator means;

loading means at the bottom of said first chamber for delivering sheets to the trays at the beginning of their ascent;

unloading means at the bottom of said second chamber for removing sheets from the trays at the end of their descent;

first blower means communicating with said upper section for aspirating air therethrough from the interior of said housing and driving the aspirated air through a heating zone;

first jet means in said first chamber trained upon the ascending trays and connected to said heating zone for receiving hot air therefrom under pressure of said first blower means;

second blower means for aspirating cold air from the surrounding atmosphere;

second jet means in said second chamber trained upon the descending trays and connected to said second blower means; and

vapor-exhaust means communicating with said lower section.

10. A drying oven for printed sheets, comprising:

a housing including an upright frame provided with a vertical partition dividing the interior thereof into a first and a second chamber;

ascending elevator means in said first chamber and descending elevator means in said second chamber each provided with supports for sheet-carrying trays;

upper horizontal transport means at the top of said frame for carrying trays from said ascending elevator means to said descending elevator means;

lower horizontal transport means at the bottom of said housing for carrying trays from said descending elevator means to said ascending elevator means;

loading means at the bottom of said first chamber for delivering sheets to the trays at the beginning of their ascent;

unloading means at the bottom of said second chamber for removing sheets from the trays at the end of their descent;

first blower means for aspirating air from the interior of said housing and driving the aspirated air through a heating zone;

first jet means in said first chamber trained upon the ascending trays and connected to said heating zone for receiving hot air therefrom under pressure of said first blower means;

second blower means for aspirating cold air from the surrounding atmosphere;

second jet means in said second chamber trained upon the descending trays and connected to said second blower means; and

third blower means for aspirating atmospheric air and discharging same downwardly from the top of said frame for holding a sheet onto a tray conveyed by said upper transport means. =l i 

1. A drying oven for printed sheets, comprising: a housing including an upright frame provided with a vertical partition dividing the interior thereof into a first and a second chamber; ascending elevator means in said first chamber and descending elevator means in said second chamber each provided with support for trays having spaced-apart sheet-carrying bars; upper horizontal transport means at the top of said frame for carrying trays in a direction parallel to said bars from said ascending elevator means to said descending elevator means; lower horizontal transport means at the bottom of said housing for carrying trays in a direction parallel to said bars from said descending elevator means to said ascending elevator means; loading means at the bottom of said first chamber for delivering sheets to the trays at the beginning of their ascent; unloading means at the bottom of said second chamber for removing sheets from the trays at the end of their descent, said unloading means including a stripper member rotatable about an axis transverse to said bars and provided with dependent teeth fitting between said bars; first jet means in said first chamber trained uPon the ascending trays and connected to a source of hot air; second jet means in said second chamber trained upon the descending trays and connected to a source of cold air; common drive means for said ascending and descending elevator means and for said upper and lower transport means; detent means yieldably engaging said stripper member for normally holding said teeth in their dependent position; and alarm means responsive to a deflection of said teeth from their dependent position by an extraneous object lodged between said bars for deactivating said drive means.
 2. A drying oven as defined in claim 1 wherein said drive means comprises a pair of ganged driving elements and a pair of driven elements respectively coupled with said driving elements for intermittent entrainment thereby in different phases of an operating cycle, one of said driven elements being coupled with said elevator means, the other of said elements being coupled with said transport means.
 3. A drying oven as defined in claim 2 wherein said driven elements are so coupled with said driving elements as to be simultaneously stationary during a fraction of an operating cycle.
 4. A drying oven as defined in claim 1 wherein said source of hot air comprises first blower means for aspirating air from the interior of said housing and driving the aspirated air through a heating zone to said first jet means, said source of cold air comprising second blower means for aspirating air from the surrounding atmosphere and driving same to said second jet means.
 5. A drying oven as defined in claim 4 wherein said partition has a hollow upper section and a hollow lower section open toward both chambers, said first blower means communicating with said upper section for aspirating air therefrom, further comprising vapor-exhaust means communicating with said lower section.
 6. A drying oven as defined in claim 4, further comprising third blower means for aspirating atmospheric air and discharging same downwardly from the top of said frame for holding a sheet onto a tray conveyed by said upper transport means.
 7. A drying oven as defined in claim 1 wherein said trays have sheet-supporting bars spaced apart in a direction said loading means includes an input conveyor with a plurality of spaced-apart endless bands fitting between the bars of said trays.
 8. A drying oven as defined in claim 1 wherein said ascending and descending elevator means each comprises two pairs of endless chains confronting each other, the chains of each pair being bridged by tray-supporting rails, said upper transport means comprising another pair of endless chains with depending lugs engageable from above with an ascending tray, said lower transport means comprising a further pair of endless chains with upstanding lugs engageable from below with a descending tray, the rails of said ascending and descending elevator means being separated by distances substantially less than the length of said rails.
 9. A drying oven for printed sheets, comprising: a housing including an upright frame provided with a vertical partition dividing the interior thereof into a first and a second chamber, said partition having a hollow upper section and a hollow lower section open toward both chambers; ascending elevator means in said first chamber and descending elevator means in said second chamber each provided with supports for sheet-carrying trays; upper horizontal transport means at the top of said frame for carrying trays from said ascending elevator means to said descending elevator means; lower horizontal transport means at the bottom of said housing for carrying trays from said descending elevator means to said ascending elevator means; loading means at the bottom of said first chamber for delivering sheets to the trays at the beginning of their ascent; unloading means at the bottom of said second chamber for removing sheets from the trays at the end of tHeir descent; first blower means communicating with said upper section for aspirating air therethrough from the interior of said housing and driving the aspirated air through a heating zone; first jet means in said first chamber trained upon the ascending trays and connected to said heating zone for receiving hot air therefrom under pressure of said first blower means; second blower means for aspirating cold air from the surrounding atmosphere; second jet means in said second chamber trained upon the descending trays and connected to said second blower means; and vapor-exhaust means communicating with said lower section.
 10. A drying oven for printed sheets, comprising: a housing including an upright frame provided with a vertical partition dividing the interior thereof into a first and a second chamber; ascending elevator means in said first chamber and descending elevator means in said second chamber each provided with supports for sheet-carrying trays; upper horizontal transport means at the top of said frame for carrying trays from said ascending elevator means to said descending elevator means; lower horizontal transport means at the bottom of said housing for carrying trays from said descending elevator means to said ascending elevator means; loading means at the bottom of said first chamber for delivering sheets to the trays at the beginning of their ascent; unloading means at the bottom of said second chamber for removing sheets from the trays at the end of their descent; first blower means for aspirating air from the interior of said housing and driving the aspirated air through a heating zone; first jet means in said first chamber trained upon the ascending trays and connected to said heating zone for receiving hot air therefrom under pressure of said first blower means; second blower means for aspirating cold air from the surrounding atmosphere; second jet means in said second chamber trained upon the descending trays and connected to said second blower means; and third blower means for aspirating atmospheric air and discharging same downwardly from the top of said frame for holding a sheet onto a tray conveyed by said upper transport means. 